a boolean expression that must be true after a constructor has created an object of the class, and that must be true after an operation has modified an object of the class

a condition that defines all valid states for an object

a condition that should always be true about a class

a condition that should never be violated in any object, after it has been created

a constraint on the state of an object that should be satisfied by every method of the class

a constraint that should be satisfied byevery method of a class

an invariant that holds for an object of the class outside of public member functions

a predicate that must be true before and after any method completes

a single (possibly quantified) logical statement about any subset of the features of an object that must be true at all times (in Eiffel, which has direct support for invariants, this only applies to times when a method is not executing)

a type of internal invariant that applies to every instance of a class at all times, except when an instance is in transition from one consistent state to another